One prior method for evaluating material is known from DE 196 13 082 C2, for example. In accordance with this prior method, imprinted material, for example sheets imprinted with bank notes or securities provided with a window thread, a hologram or a kinegram, are illuminated by an illumination device in such a way that light reflected by the imprinted material falls into a photoelectric sensor. The image taken by the photoelectric sensor can subsequently be evaluated in an evaluation device, such as, for example a standard computer, with suitable evaluation software, and this image can be checked for printing errors.
Particularly in connection with bank notes, and also in connection with other types of imprinted material, a position of identification characteristics can vary in a defined expected area and within defined tolerance levels because of previous production processes. For example, the position of a window thread in relation to a printed image of bank notes can vary on a printed sheet because of the properties of the production process for producing the window thread. With image-dependent evaluation devices of this type, it is possible that tolerance deviations of defined identification characteristics, which deviations basically are tolerable, can lead to interferences with the printed image inspection. When comparing a faultless printed pattern, such as a master with an actual printed image, one image position after the other image position are compared with each other. Position deviations of identification characteristics are detected as errors, which position deviations are not actually errors. With the method proposed in DE 196 13 082 C2, this problem is solved through the use of suitable illumination steps, so that the identification characteristics, whose positions vary, reflect the light emitted from the illumination device so strongly that these areas are recognized by the electronic evaluation devices because of their high degree of reflection and can be further processed. However, a prerequisite for such an evaluation, in accordance with this prior process, is that the identification characteristics, whose position varies, must have a sufficient reflection capacity. For example these identification characteristics are embodied as silver threads.
It is a limitation of the described prior method that identification characteristics, whose image properties do not sufficiently differ from the image properties of the remaining image, when recorded by a photoelectric sensor, such as is the case with colored window threads, for example, cannot be detected with sufficient dependability by the evaluation device.
DE 100 00 364 A1 describes a method for detecting errors, which is based on characteristics. A mean background level is calculated and is subtracted for amplifying the properties of certain characteristics.
A method for checking the print quality of printed images is known from DE 26 20 767 C2. The partial images resulting from different printing processes are combined into a total pattern.
U.S. Pat. No. 5,125,037 A1 discloses a method for controlling printed images in which additional markers are imprinted. The position and density of these print markers is checked.
It is known from U.S. Pat. No. 6,240,218 to employ projections for accelerating the image comparison method.
JP 11-185039 A describes a method for determining the position of an identification characteristic. A background reference value and a mask reference are stored in an evaluation device. The actual position of the identification characteristic is determined by correlating the reference image with a difference image.
U.S. Pat. No. 5,524,069 discloses a method for separating the foreground and the background of images.